In recent years, development research has been carried out actively on white light emitting devices with a blue light emitting element as a light emitting source. In particular, white light emitting diodes using a blue light emitting diode element are light in weight, use no mercury, and are long in lifetime, and the demand for the diodes are thus expected to be hence expanded rapidly. It is to be noted that a light emitting device using a light emitting diode element as a light emitting element is referred to as a light emitting diode. In the most commonly adopted method as a method for converting blue light from a blue light emitting diode element to white light, a white color is obtained in a pseudo fashion by color mixing with a yellow color in complementary relationship with a blue color. As described in, for example, Patent Literature 1, a white light emitting diode can be configured in such a way that a coating layer containing a phosphor which partially absorbs blue light to emit yellow light is provided on the front of a light emitting diode element which emits blue light, and a mold layer or the like is provided ahead which mixes blue light from the light source with yellow light from the phosphor. As the phosphor, a YAG(Y3Al5O12) powder activated cerium (hereinafter, referred to as YAG:Ce) or the like is used.
However, in the case of the structures of now commonly used white light emitting diodes typified by Patent Literature 1, a powdered phosphor is mixed with a resin such as epoxy, and applied, thus, it is difficult to ensure homogeneity in the mixing state of the powdered phosphor and the resin, and control stabilization or the like of the thickness of the applied film, and it is pointed out that the white light emitting diodes are likely to cause color unevenness or variability. In addition, the resin required in the case of using the powdered phosphor is inferior in heat resistance as compared with metals or ceramics, and thus likely to cause a decrease in transmittance due to the resin modified by heat from the light emitting element. Therefore, this decrease in transmittance is problematic for higher outputs of white light emitting diodes required now.
The inventors have proposed a ceramic composite for light conversion, which is composed of a solidification product formed from continuously and three-dimensionally mutually entangled multiple oxide phases including a YAG:Ce fluorescent phase and an Al2O3 phase, and a white light emitting device configured with the use of a blue light emitting element and the ceramic composite for light conversion (Patent Literature 2). The ceramic composite for light conversion can stably achieve homogeneous yellow fluorescence because the YAG:Ce fluorescent phase is distributed homogeneously, and has excellent heat resistance because of being a ceramic. In addition, the ceramic composite itself is a bulk body, and thus requires no resin for constituting the white light emitting device, unlike in Patent Literature 1. For this reason, the white light emitting device is reduced in color unevenness or variability, and preferred for higher outputs.
In the case of a white light emitting device using a blue light emitting diode element and a YAG:Ce phosphor, light from the blue light emitting diode element commonly used now has a peak wavelength around 460 nm for blue (for example, CIE 1931 chromaticity coordinates (hereinafter, chromaticity coordinates) Cx=0.135, Cy=0.08). This is because the luminous efficiency of the YAG:Ce phosphor is high in this wavelength range. By the way, the fluorescence color of a YAG:Ce (hereinafter, non-adjusted YAG:Ce) phosphor which has an untuned emission wavelength is yellow (for example, chromaticity coordinates Cx=0.41, Cy=0.56) with a peak wavelength around 530 to 545 nm. For this season, color mixing of the light around 460 nm from the blue light emitting diode element and the light from the non-adjusted YAG:Ce phosphor results in a deviation from a white color (for example, 6000K:chromaticity coordinates Cx=0.32, Cy=0.34) toward a green side. Therefore, in order to achieve a white color from this configuration, there is a need to use a YAG:Ce phosphor with a fluorescence peak wavelength on the further red side (longer wavelength side).
As for a YAG:Ce phosphor, it is known as known art that the increased content of Ce as an activator can shift the peak of the fluorescence wavelength to the longer wavelength side (Non-Patent Literature 1). Thus, the peak of the fluorescence wavelength of the YAG:Ce phosphor can be transferred to around 560 nm.
In addition, as for a YAG:Ce phosphor, it is known as known art that, for example, the Y element partially substituted with a Gd element can shift the peak of the fluorescence wavelength to the longer wavelength side (Non-Patent Literatures 2 and 3). Patent Literature 2 proposes that a YAG:Ce phosphor with a fluorescence wavelength thus tuned to the longer wavelength side is combined with a blue light emitting diode element to constitute a white light emitting diode, and thereby achieve a white color (CIE chromaticity coordinates Cx=0.33, Cy=0.33).
In the case of the ceramic composite for light conversion as described in Patent Literature 2, the inventors also demonstrate that the adjustment of the composition of the entire solidification product can tune the peak of the fluorescence wavelength in the range of 550 to 560 nm or 540 to 580 nm (Patent Literature 3).